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United States Patent |
5,159,300
|
Nakamura
,   et al.
|
October 27, 1992
|
Noise filter comprising a monolithic laminated ceramic varistor
Abstract
A noise filter of a monolithic type in which a common electrode is formed
in a sintered body having varistor characteristics so as to extend from a
first portion to a second portion of side surfaces of the sintered body,
and at least one through electrode is formed in a position at a height
spaced apart from the common electrode through a sintered body layer in
the direction of thickness so as to extend from a third portion to a
fourth portion of the side surfaces of the sintered body and to intersect
the common electrode.
Inventors:
|
Nakamura; Kazutaka (Nagaokakyo, JP);
Yoneda; Yasunobu (Nagaokakyo, JP);
Sakabe; Yukio (Nagaokakyo, JP);
Sakamoto; Yukio (Nagaokakyo, JP);
Yamamoto; Hidetoshi (Nagaokakyo, JP);
Sakai; Seiji (Nagaokakyo, JP)
|
Assignee:
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Murata Manufacturing Co. Ltd. (JP)
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Appl. No.:
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549221 |
Filed:
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July 6, 1990 |
Foreign Application Priority Data
Current U.S. Class: |
333/184; 333/185; 338/21 |
Intern'l Class: |
H01C 007/10; H01P 007/00 |
Field of Search: |
333/12,181,184,185
338/20,21,204
361/306,111,321
|
References Cited
U.S. Patent Documents
4212045 | Jul., 1980 | Martzloff | 338/21.
|
4290041 | Sep., 1981 | Utsumi et al. | 338/21.
|
4746557 | May., 1988 | Sakamoto et al. | 333/184.
|
4785276 | Nov., 1988 | May | 338/21.
|
4920328 | Apr., 1990 | Hayashi et al. | 338/21.
|
5034709 | Jul., 1991 | Azumi et al. | 333/185.
|
Foreign Patent Documents |
0285906 | Nov., 1988 | JP.
| |
0296316 | Dec., 1988 | JP | 338/21.
|
0107511 | Apr., 1989 | JP.
| |
Primary Examiner: LaRoche; Eugene R.
Assistant Examiner: Ham; Seung
Attorney, Agent or Firm: Ostrolenk, Faber, Gerb & Soffen
Claims
What is claimed is:
1. A noise filter comprising:
a monolithic cofired multilayer sintered body comprising a plurality of
laminated ceramic layers having varistor characteristics and a set of
electrodes separated by said layers;
said sintered body having a pair of main surfaces which define a direction
of thickness therebetween, and an outside surface extending between said
main surfaces;
said set of electrodes comprising:
a common electrode formed in the sintered body so as to extend from a first
portion to a second portion of said outside surface of the sintered body;
and
at least one through electrode spaced apart from said common electrode by
at least one ceramic layer in the direction of thickness and extending
from a third portion to a fourth portion of the outside surface of said
sintered body and crossing said common electrode.
2. The noise filter according to claim 1, wherein a plurality of said
through electrodes are formed.
3. The noise filter according to claim 2, wherein said through electrodes
adjacent to each other are formed in positions at different heights in the
sintered body.
4. The noise filter according to claim 2, wherein said plurality of through
electrodes are arranged in a distributed manner on sintered body layers
positioned on both sides of the common electrode.
5. The noise filter according to claim 4, wherein the adjacent through
electrodes are formed in positions at different heights.
6. The noise filter according to claim 1, wherein said through electrode is
constituted by a plurality of through electrode portions overlapped with
each other separated by a sintered body layer.
7. The noise filter according to claim 1, wherein a plurality of said
common electrodes are formed.
8. The noise filter according to claim 1, wherein the first and second
portions of the outside surfaces of said sintered body are a pair of side
surfaces opposed to each other of the sintered body, and the third and
fourth portions thereof are another pair of side surfaces opposed to each
other of the sintered body.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a noise filter constructed of a nonolithic
sintered body obtained by laminating a plurality of ceramic green sheets
having varistor characteristics which separate a set of electrodes,
followed by cofiring.
2. Description of the Prior Art
In recent years, the development of microcomputers has resulted in
microcomputers being incorporated in various apparatus such as industrial
equipment, household electrical appliances and communications equipment.
Equipment carrying a microcomputer for digital control processing is liable
to be destroyed or erroneously operated due to noise. An accident can
actually happen due to noise. In many cases, the path of entrance of noise
into the equipment is a power supply portion and a signal interconnection
portion. Consequently, the function of a noise filter used in an
input/output portion is taken seriously.
In order to solve the problem of EMI noise, it is necessary that the noise
does not go in and out of the equipment. The noise normally goes in and
out of the equipment through an electric wire. Consequently, a method of
connecting a noise filter to the electric wire in the vicinity of the
input/output portion of the equipment has been most commonly used.
As noise filters conventionally used, the following noise filters have been
known: (a) one using a capacitor, (b) the capacitor-inductor composite
system, (c) one using an inductor, and (d) one using a varistor.
The noise filter using a capacitor (a) is superior in that it reduces very
small noise but cannot absorb a high-voltage pulse such as a static charge
by only the capacitor. Accordingly, the destruction and the maulfunction
of an apparatus or equipment incorporating the noise filter based on
high-voltage pulse noise cannot be prevented.
The noise filter of the capacitor-inductor composite system (d) is suitable
for absorption of white noise or the like. In addition, the noise filter
using a varistor (d) is suitable for absorption of transient noise.
However, absorption of noise on a signal line is closely related to the
frequency of a signal. Further, since digitization has recently proceeded,
large distortion must not occur in a waveform. Therefore, the noise filter
cannot be used as a noise filter for a signal line if its capacitance is
too large. In addition, the withstanding voltage of an IC is approximately
60 to 70 volts. Accordingly, a noise component must be suppressed to 60 to
70 volts or less.
The noise filter of the capacitor-inductor system has no problem in terms
of its capacitance. However, there is a problem that the transient noise
in a resonance frequency band is passed without any modification, thereby
to make it impossible to protect a circuit. Furthermore, in the case of
high-voltage noise, the waveform can be changed but its peak value cannot
be greatly restrained.
In the noise filter using a varistor, on the other hand, if its capacitance
is 500 pF or more and a signal of hundreds of kilohertzs to tens of
megahertzs such as a signal which is passed on a signal line is handled,
the signal per se may be absorbed or deformed. Presently, therefore, a
varistor is not commonly used as a device for reducing noise on the signal
line.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a noise filter whose
capacitance is reduced, thereby to make it difficult to absorb and deform
a signal, while taking advantage of characteristics of a varistor superior
in absorption of transient noise.
The present invention provides a noise filter superior to a filter of the
capacitor-inductor system as a noise filter for a signal line by paying
attention to dielectric characteristics of a varistor and effectively
utilizing the characteristics.
The noise filter according to the present invention is constructed by
utilizing a sintered body obtained by laminating a plurality of ceramic
green sheets having varistor characteristics through electrodes, followed
by cofiring. More specifically, a comon electrode is formed in the
sintered body so as to extend from a first portion to a second portion on
side surface of the sintered body having varistor characteristics. At
least one through electrode is formed in a position at a height spaced
apart from the common electrode through a sintered body layer in the
direction of thickness so as to extend from a third portion to a fourth
portion on the side surfaces of the sintered body and to intersect the
above common electrode.
According to the present invention, the noise filter is constructed by
utilizing a sintered body obtained by laminating ceramic green sheets
having varistor characteristics through the electrodes, followed by
cofiring. Consequently, the noise filter is of a monolithic type and thus,
a varistor voltage can be lowered, thereby to make it possible to enhance
the circuit protecting effect. In addition, the through electrode or
electrodes are formed so as to intersect the common electrode.
Accordingly, the electrode-overlapping area in a varistor characteristic
portion can be decreased, thereby to make it possible to lower the
capacitance of the noise filter. Moreover, the varistor characteristic
portion is of a distribution type. Accordingly, the unnecessary inductance
is decreased, thereby to improve the varistor characteristics.
Therefore, a noise filter has small capacitance and both noise
absorbabilities due to a capacitor and a varistor, thereby to make it
possible to effectively absorb noise in a wide variety of forms such as
white noise and transient noise. Accordingly, the malfunction and the
destruction of an apparatus or equipment incorporating the noise filter
can be effectively prevented.
The foregoing and other objects, features, aspects and advantages of the
present invention will become more apparent from the following detailed
description of the present invention when taken in conjunction with the
accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an exploded perspective view for explaining ceramic green sheets
used for obtaining a first embodiment of the present invention and the
shapes of electrodes formed thereon;
FIG. 2 is a perspective view showing a sintered body obtained in the first
embodiment of the present invention;
FIG. 3 is a perspective view showing the appearance of a noise filter
according to the first embodiment of the present invention;
FIG. 4 is a circuit diagran showing the noise filter according to the first
embodiment of the present invention;
FIG. 5 is a diagram showing a circuit used for measuring the noise filter;
FIG. 6 is a diagram for explaining noise absorbability of the noise filter
according to the first embodiment of the present invention and a noise
filter in a comparative example;
FIG. 7 is an exploded perspective view for explaining ceramic green sheets
used in a second embodiment of the present invention and the shape of
electrodes formed thereon;
FIG. 8 is an exploded perspective view for explaining ceramic green sheets
used in a third embodiment of the present invention and the shapes of
electrodes formed thereon;
FIG. 9 is a perspective view showing a noise filter according to the third
embodiment of the present invention;
FIG. 10 is a perspective view showing a modified example of the noise
filter according to the third embodiment:
FIG. 11 is a perspective view for explaining a modified example in which a
common electrode is arranged below through electrodes; and
FIGS. 12A to 12E are plan views for explaining ceramic green sheets used in
a fouth embodiment of the present invention and the shapes of electrodes
formed thereon, respectively.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Raw materials are weighed so as to have composition containing 97.5 mol %
of ZnO, 0.5 mol % of Bi.sub.2 O.sub.3, 0.5 mol % of Co.sub.2 O.sub.3, 0.5
mol % of MnO, and 0.5 mol % of Sb.sub.2 O.sub.3 and water is added to the
raw materials, followed by grinding by a ball mill.
Slurry obtained in the above described manner is dehydrated and dried. Thus
obtained dried powder is calcined for two hours at a temperature of
780.degree. C.
The calcined materials are ground with an organic binder. Further ethyl
alcohol is added thereto as a solvent such that the materials take the
form of slurry. A green sheet having a thickness of 60 .mu.m is obtained
using this slurry by the Doctor blade process.
The green sheet obtained is punched to be of a predetermined size and
shape, to obtain ceramic green sheets 1 to 4 as shown in FIG. 1.
Conductive pastes containing Ag and Pd to have the weight ratio of 7:3 are
applied on upper surfaces of the ceramic green sheets 2 and 3, to form
through electrode portions 5a to 5c and a common electrode portion 6.
Description is now made by assigning the same reference numerals as those
of the common electrode portion and the through electrode portions to a
common electrode and through electrodes formed after sintering.
Three ceramic green sheets identical to the ceramic green sheets 1 to 4 are
respectively laminated on upper and lower surfaces of the ceramic green
sheets 1 and 4, followed by applying pressure of 2 ton/cm.sup.2.
A laminated body chip obtained is sintered for two hours at a temperature
of 1100.degree. C., to obtain a sintered body 7 shown in FIG. 2.
In the sintered body 7, a common electrode 6 is formed between a first side
surface 7a and a second side surface 7b which are opposed to each other.
Similarly, three through electrodes 5a, 5b and 5c are formed so as to
extend between a third side surface 7c and a fourth side surface 7d and to
intersect the common electrode 6.
Conductive pastes containing Ag and Pd at a weight ratio of 7:3 are then
printed in portions, which are exposed to the side surfaces of the
sintered body 7, of the above through electrodes 5a to 5c and the above
common electrode 6 and are baked for thirty minutes at a temperature of
1000.degree. C., thereby to form outer electrodes. Thus obtained noise
filter having the outer electrodes is shown in FIG. 3.
In FIG. 3, both ends of the through electrodes 5a to 5c (see FIG. 2) are
electrically connected to outer electrodes 8a, 8b, 9a, 9b, 10a and 10b,
respectively. Both ends of the common electrode 6 (see FIG. 2) are
electrically connected to outer electrodes 11a and 11b.
The widths of the through electrodes 5a to 5c are set to 0.7 mm, the width
of the common electrode 6 is set to 1.0 mm and the spacing between the
through electrodes 5a to 5c is set to 1.2 mm.
FIG. 4 illustrates a circuit structure of a noise filter obtained.
Characteristics of the noise filter obtained in the above described manner
according to the present embodiment are shown in the following table 1. In
the table 1, the electrodes 8a to 10a described below an item "between
common electrode and through electrode" are respectively characteristics
between the outer electrode 11a electrically connected to the common
electrode 6 and the outer electrode 8a, 9a and 10a. In addition, 8a-9a and
9a-10a described below an item "between through electrodes" are
respectively characteristics between the outer electrodes 8a and 9a and
between the outer electrodes 9a and 10a which are connected to the through
electrodes 5a to 5c.
TABLE 1
______________________________________
between common electrode
and through electrode between through
electrode electrode
electrode
electrodes
8a 9a 10a 9a-10a 8a-9a
______________________________________
varistor
13.2 12.9 13.1 26.4 26.3
voltage (V)
capacitance
124 127 125 60 58
(pF)
.alpha. 24.7 26.5 25.6 32.1 31.6
______________________________________
The varistor voltage is measured by applying a current flow of 0.1 mA to
the noise filter obtained in the present embodiment. The voltage
non-linearity index .alpha. is calculated from the following equation,
where its voltage values of V.sub.0.1 and V.sub.1 are at 0.1 mA and 1 mA,
respectively.
.alpha.=1/log (V.sub.1 /V.sub.0.1)
A measuring circuit shown in FIG. 5 is used, and the above described noise
filter is inserted thereinto as a sample. A pulse of 1 kilovolts is
applied between the outer electrodes 10b and 11a of the noise filter, to
observe the change in waveform at that time. For comparison, the same
pulse is applied to a filter using a capacitor comprising an ordinary
dielectric ceramic chip and a filter of a disk-shaped 20-volts varistor,
to observe the change in waveform at that time. The changes in waveform in
the noise filter in the embodiment and in the noise filter in the
comparative examples are shown in FIG. 6.
As can be seen from FIG. 6, in the noise filter according to the present
embodiment, pulse-shaped noise can be effectively restrained.
Although in the above described embodiment, three through electrodes are
formed as through electrode portions 5a to 5c, a noise filter may be of
such construction that only one through electrode portion 5b is arranged
below a common electrode portion 6, as shown in FIG. 7. More specifically,
in the present invention, the number of through electrodes is arbitrary.
FIG. 8 is an exploded perspective view for explaining ceramic green sheets
used in a noise filter according to a third embodiment of the present
invention and the shapes of electrodes formed thereon. Ceramic green
sheets 21 to 23 mainly composed of ceramic materials exhibiting varistor
characteristics are prepared. A common electrode 26 is formed on the
central ceramic green sheet 22 in parallel with a long side of the ceramic
green sheet 22. On the other hand, through electrodes 25a and 25c and 25b
and 25d are respectively formed on upper surfaces of the ceramic green
sheets 21 and 23 in parallel with short sides of the ceramic green sheets
21 and 23. The through electrode 25b is formed so as to be positioned at
the center between the upper through electrodes 25a and 25c, and the
through electrode 25c is formed so as to be positioned at the center
between the lower through electrodes 25b and 25d.
The above described ceramic green sheets 21 to 23 are laminated in the
direction as shown and ceramic green sheets having no electrodes formed
thereon are laminated above the ceramic green sheet 21 and are sintered,
thereby to obtain a sintered body 27 shown in FIG. 9. Outer electrodes 28a
to 31a, 28b to 31b, 32a and 32b are respectively formed on side surfaces
of this sintered body 27. In the noise filter according to the present
embodiment, noise filter units are respectively constructed between the
outer electrodes 28a to 31a and the outer electrodes 28b and 31b.
Moreover, in the present embodiment, through electrodes adjacent to each
other in the traverse direction, for example, the through electrodes 25a
and 25b are formed in positions at different heights in the sintered body
27. Accordingly, crosstalk between the adjacent noise filter units can be
effectively restrained.
Meanwhile, as shown in FIG. 10, the through electrodes respectively
constituting the noise filter units may be composed of two through
electrode portions 25a and 25a', 25b and 25b', 25c and 25c', and 25d and
25d' which are overlapped with each other through a sintered body layer,
respectively.
Furthermore, as shown in FIG. 11, a common electrode 26 may be arranged
below the positions at heights where the through electrodes 25a to 25d are
formed. However, it is preferable that the adjacent through electrodes,
for example, the through electrodes 25a and 25b are arranged on opposite
sides of the common electrode 26, as shown in FIGS. 8 and 9. The reason
for this is that mutual inference can be effectively prevented by
arranging the common electrode 26 connected to an earth potential between
the adjacent through electrodes.
Additionally, it is not always necessary to form the through electrodes 25a
to 25d every other one on the upper surfaces of the two ceramic green
sheets, as the embodiment shown in FIG. 8, so as to arrange the through
electrodes constituting the adjacent noise filter units in positions at
different heights. For example, adjacent through electrodes 25a and 25d
may be formed in a distributed manner on three ceramic green sheets, for
example, ceramic green sheets 41, 43 and 45 shown in FIGS. 12A, 12C and
12E, thereby to make different the positions at heights where adjacent
through electrodes are arranged.
Furthermore, in the example shown in FIGS. 12A to 12E, ceramic green sheets
42 and 44 respectively having common electrodes 26 formed thereon and
shown in FIGS. 12B and 12D are inserted between the ceramic green sheets
41, 43 and 45. Thus, a plurality of common electrodes may be formed in a
sintered body.
Additionally, a plurality of common electrodes may be formed on one ceramic
green sheet.
As described in the foregoing, according to the present invention, a
plurality of through electrodes may be formed at positions at different
heights, provided that the through electrodes are spaced apart from a
common electrode through a sintered body layer and are formed so as to
intersect the common electrode.
Moreover, the common electrode and the through electrodes need not be
necessarily formed so as to respectively extend between different side
surfaces of the sintered body in the example as shown but can be formed so
as to extend between arbitrary two portions on outer peripheral side
surfaces of the sintered body. More specifically, both ends of the common
electrode or the through electrodes may be exposed to one side surface of
the sintered body.
Although the present invention has been described and illustrated in
detail, it is clearly understood that the same is by way of illustration
and example only and is not to be taken by way of limitation, the spirit
and scope of the present invention being limited only by the terms of the
appended claims.
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